Yea, I'm not really buying that whole story - that he knew it was Molly from the start, but proposed to Melissa just for the "fun of it." Not saying that's not how it went down, but I don't buy that it was planned by anyone from the start.
And as for the "friends" thing w/Jill - I actually get what I think he was saying. His problem with Jill was that he felt there was no romance, that there was only a friendship. When she said her grandparents were best friends, he seemed to take that to mean that's all she (Jill) wants/needs. Whereas he wanted the romance and passion along with the best friends thing. And I agree! That's what I want too. And since he struggled to feel that part with her, he ended it. At least that was my take on it all.